A spatially variable optical filter has a transmission wavelength varying in a transverse direction across the filter. A compact optical spectrometer can be constructed by attaching a photodetector array to a spatially variable optical filter. A filter having the transmission wavelength varying linearly with distance in a transverse direction across the filter is called a linearly variable filter (LVF). Linear variation of the transmission wavelength with distance is convenient, although not necessary. Optical spectra obtained using an LVF and a constant-pitch photodetector array have a constant wavelength step.
Pellicori et al. in U.S. Pat. No. 4,957,371 disclose a wedge-filter spectrometer including a LVF having a first plurality of layers of high index of refraction material and a second plurality of layers of low index of refraction material, individual high- and low-index layers overlapping each other and having a substantially linearly tapered thickness, to form a linearly variable optical thin film interference filter. A photodetector array is attached to the LVF, resulting in a very compact overall construction.
Anthon in U.S. Pat. No. 6,057,925 discloses a compact spectrometer device including a thin film interference LVF and a photodetector array coupled to the LVF via an array of gradient-index lenses or an array of microlenses, for use in a color sensing device, such as a portable colorimeter. Lightweight and robust construction of the thin film interference LVF-based spectrometer allows the portable colorimeter to characterize color of articles in field conditions.
Weigl et al. in U.S. Pat. No. 6,091,502 disclose a compact LVF-based spectrometer for performing fluorescence and absorption spectral measurements in flow cells with spatial resolution. By placing the LVF in an optical path, such that the transmission variation of the filter occurs in the flow direction, it is possible to spectroscopically determine concentration of dye markers of proteins in a flow of biological cells.
Referring to FIG. 1A, a typical prior-art compact optical spectrometer 100, similar to those used in Pellicori, Anthon, and Weigl devices, includes a LVF 102 optically coupled to a photodetector array 104. Transmission wavelength λT varies in a direction 106 across the LVF 102. In operation, light 108 impinges onto the LVF 102. The LVF 102 passes through only a narrow wavelength band around the transmission wavelength λT, which varies in the direction 106 parallel to the photodetector array 104. As a result, each photodetector 105 of the photodetector array 104 is responsive to a different wavelength band of the light 108. By measuring photocurrents of each photodetector 105 of the photodetector array 104, an optical spectrum of the light 108 can be obtained.
The LVF 102 includes a thin film stack 112 supported by a substrate 110. Referring to FIG. 1B, the thin film stack 112 includes two regions: a block region 121 for blocking wavelengths shorter than and longer than λT, and a bandpass region 122 for transmitting only a narrow passband centered around λT. Each of the two regions 121 and 122 includes alternating high-index layers 131 and low-index layers 132 having high and low refractive indices, respectively. The materials of the high-index 131/low-index 132 layers are the same across the regions 121 and 122, only the thicknesses are varied to achieve the optical performance required. The blocking region 121 includes quarter-wave stacks for blocking wavelengths other than λT, and the bandpass region 122 half-wave stacks for transmitting the narrow passband centered around λT. The material combinations in the material pair can include metal oxides or fluorides.
One drawback of the LVF 102 is an inherent tradeoff between optical performance of the LVF 102 and the overall thickness of the thin film stack 112. To ensure good blocking of the wavelengths other than λT, the blocking region 121 has to include many layers. For low-loss oxides, the number of layers can be up to a hundred layers. To ensure narrow passband around λT, the bandpass region 122 also needs to include many layers, and/or to include a thick central layer. Large thickness of the thin film stack 112 results in an increase of internal stresses in the thin film stack 112, causing it to break and/or delaminate from the substrate 110. High-index material, such as silicon, can be used to reduce the overall number of layers. However, high-index materials typically increase optical loss of the LVF 102.